This invention relates to an artificial joint and more particularly to anchors of the artificial joint, the anchors being inserted into a bone.
There have been proposed and used various structures of joint portions for artificial articulations, wherein a spherical joint end is used in combination with an associated concave joint end; joint ends having through holes are coupled by a pin; and an elastic material is used for the joint portion.
In the structure in which a spherical joint end is used together with a concave joint end, a high accuracy in configuration is needed for smooth movement. Also, a manufacturing process thereof is time-consuming and needs a great amount of labor. One of the joint ends is rotated in a universal direction relative to the other so that it is impossible to apply such joint structure to an articulation where two components should be rotated in a single plane as in a finger joint.
In the structure of pin-joint, the number of necessary components such as pins is increased and in assembling work for embedding the joint structure in the patient's articulation needs a long period of time. In addition, a large load is applied to the thin pin, resulting in reduction in durability and wear resistance.
In the elastic structure of joint, the movement of the joint is limited and it is necessary to keep the joint in a folded state with a continuous force.
On the other hand, there has been used an artificial joint having two anchors each inserted into a bone, the anchors being coupled having an angle changeable to each other. The anchors may be cylindrical, square or bar-shaped and generally have smooth surfaces (Japanese Pat. Appln. Laid-Open Nos. 54-127197, 52-30096, 50-18197). The smooth surface allows the anchor inserted to move in the direction of insertion or opposite thereto and connection with the bone is slow. Consequently, the bone must be screwed laterally and this requires time and labor for such an operation.
There has also been proposed an anchor having a boss on the surface to solve the above problem (Japanese Pat. Appln. Laid-Open No. 56-130148). In FIG. 9, there is shown an artificial joint comprising an inner male member 41 having two parts, a first curved portion 42 for engaging with a female part and a second anchor portion 43 having bosses 44 consisting of faces 44a directed to the curved portion 42 and faces 44b directed opposite thereto. The face 44a is substantially perpendicular to the axis of the anchor 43, thus ensuring that the anchor 43 is hard to draw out of a bone 45.
An artificial joint of the above-described sort cannot be fixed until the tissue of the bone 45 is completely coupled to the surface of the anchor 43. However, a long period of time is needed for the bone to completely couple to the anchor portion 43, similarly to a member whose anchor has a smooth surface